The present invention relates to an induction motor drive apparatus, and more particularly to and induction motor drive apparatus that allows smooth rotation of an induction motor in a low-speed region.
The speed of three-phase induction motors are dependent upon the frequency and effective value of the three-phase AC voltages applied thereto, and therefore can be controlled by varying these two quantities. Drive systems for three-phase induction motors generally employ an inverter circuit which converts a DC voltage into a three-phase AC voltage. The frequency and effective value of this three-phase AC voltage can be made variable by controlling the on-off timing of the power transistors that constitute the inverter circuit. Accordingly, if it is desired to vary the speed of an induction motor, the effective value and/or frequency of the three-phase AC voltage acquired from the output of the inverter circuit need only be changed by suitable control of the inverter circuit. More specifically, with the induction motor operating in a high-speed region, speed is controlled by varying only the frequency of the three-phase AC voltage from the inverter circuit while holding the effective value of the three-phase AC voltage constant. This is known as variable frequency control. With the motor operating in the low speed region, speed is controlled by varying the frequency of the three-phase AC voltage and by adjusting the effective value thereof in accordance with the change in frequency. This is known as variable voltage-variable frequency control. The effective value of the voltage is not varied in the high speed region because the commercial power source voltage is already determined and because of limitations in the power transistors constituting the inverter circuit to withstand voltages.
The variable voltage-variable frequency control applied in the low speed region is an effective method of speed control since the speed of the induction motor can be regulated while holding the generated torque constant. However, torque irregularity or ripple as well as excitation noise arise when the speed of an induction motor is lowered by the variable voltage-variable frequency control method with the induction motor operating in the low speed region. This is attributable to the fact that a smooth revolving magnetic field is not generated because the three-phase AC voltage waveform on the output side of the inverter circuit is not perfectly sinusoidal but is rather an uneven stepped waveform which is only an approximation of a sinusoidal wave. In particular, an abrupt decrease in load to a small value when the motor is in the low speed region makes smooth rotation of the motor impossible and thus results in intermittent rotation because of torque irregularity, a decrease in the fly-wheel effect owing to the smaller load, and the fact that the generated torque becomes greater than the torque required for the particular load.
This torque irregularity and excitation noise can be considerably suppressed and smooth rotation made possible by making the effective value of the three-phase AC voltage smaller than a stipulated value decided on the basis of the variable voltage-variable frequency control. However, if the load increases under a condition in which the effective value of the three-phase AC voltage is smaller than the stipulated value, the induction motor will not be able to generate the required torque, and the motor will come to a stop. Hence, there is a need for an induction motor drive system that can eliminate torque irregularity and excitation noise in the low speed region and yet permit smooth rotation of an induction motor at a command speed, even under sudden changes in load.